Design and Experiments with High Power Microwave Sources: The Virtual Cathode Oscillator

Möller, Cecilia

KTH, School of Electrical Engineering (EES), Space and Plasma Physics.

2012 (English)Doctoral thesis, comprehensive summary (Other academic)

Abstract [en]

High-Power Microwaves (HPM) can be used to intentionally disturb or destroy electronic equipment at a distance by inducing high voltages and currents.This thesis presents results from simulations and experiments with a narrow band HPM source, the vircator. The high voltages needed to generate HPM puts the vircator under great stress, especially the electrode materials. Several electrode materials have been tested for endurance and their influence on the characteristics of the microwave pulse. With the proper materials the shot-to-shot variations are small and the geometry can be optimized in terms of e.g. output power or frequency content. Experiments with a resonant cavity added to the vircator geometry showed that with proper tuning of the cavity, the frequency content of the microwave radiation is very narrow banded and in this case the highest fields are generated. The vircator can be built in different geometries. Four different vircator types are investigated and the coaxial vircator is found to have advantages as a high radiated power and the possibility to vary the polarization during operation.Since HPM pulses are very short and have high field strengths, special field probes are needed. An HPM pulse may shift in frequency during the pulse and therefore it is very important to be able to compensate for the frequency dependence of the entire measurement system. The development and use of a far-field measurement system is described.

Abstract [en]

Repetitive use of a high-power microwave (HPM) radiation source implies strong erosion on cathode and anode materials. Electrode material endurance has been studied in a series of experiments with an axial vircator powered by a compact Marx generator. The Marx generator operated in a 10 Hz repetitive mode with a burst of ten pulses. Velvet and graphite was used as electron-emitting materials, and they showed markedly different pulse characteristics. Three different anode materials were used; stainless steel mesh, stainless steel wires and molybdenum wires, which all had different influence on the pulse characteristics.

Abstract [en]

Experiments on an axial virtual-cathode oscillator (vircator) with a resonance cavity enclosing the virtual cathode are reported. The vircator is driven by a repetitive Marx generator operating in a single-shot mode. To be able to separate different radiation mechanisms, the design of the vircator allows adjustment of the cavity depth as well as the way microwave radiation is extracted. The microwave radiation is measured with a pair of free-field B-dot sensors. The maximum field strengths were registered when the bandwidth was very narrow.

Nyholm, Sten

Abstract [en]

An initial experimental study of a vircator where a feedback mechanism is used to premodulate the electron beam has been performed. The anode-cathode gap distance and the applied voltage were varied and their influence on the frequency content and field strength of the generated microwave pulse are investigated and compared with particle in cell simulations. The frequency content of the microwave radiation was in good agreement between experiments and simulations. The vircator was very narrow banded at 2 GHz.

Larsson, Anders

Nyholm, Sten

Abstract [en]

Studies of the concept of direct excitation of the TE11 mode in a coaxial vircator using three-dimensional particle-in-cell simulations is presented. The excitation of the TE11 mode is made possible by sectioning the emitting surface of the cathode, thus creating two interacting virtual cathodes. It is shown that the two virtual cathodes formed oscillate in push-pull mode as previously has been described for a planar geometry.

Larsson, Anders

Nyholm, Sten E.

Abstract [en]

Experiments on a coaxial vircator with a sectioned emitter are reported. The emitting area is sectioned to form two opposing emitters in order to favor growth of the TE11 mode and inhibit growth of the TM01 mode that is usually excited in a coaxial vircator. Experiments are performed using a compact 320-J 400-kV Marx generator and a compact coaxial vircator built in a standard 8 '' vacuum tube. The radiated magnetic-field strength is measured by means of four free-field (B-dot) probes, and experiments show that sectioning the emitter does, in fact, lead to generation of the TE11 mode.

Elfsberg, Mattias

Hurtig, Tomas

Möller, Cecilia

Nyholm, Sten E.

Abstract [en]

Experiments on a coaxial vircator have been performed. A vircator is a narrow band high power microwave source without any external generated magnetic field [1]. A coaxial vircator is an advantageous design of a microwave radiation source for a compact HPM-system. With a limited size and outer diameter it is possible to use a larger emitting area compared to an axial design. A conventional coaxial vircator will generate the radiation in TM01 mode, due to its geometrical properties. For a compact HPM-system, radiation in TE11 mode is preferred when the radiated energy needs to be focused on a specific target. For operation in TE11 mode a sectioned emitter can be used rather than a circumcircular. The efficiency of the vircator can be greatly improved by optimizing the geometry of the vircator housing. The impedance of the pulsed power supply driving the vircator and the impedance of the vircator, depending on the A-K gap and amount of emitting material, is also important for maximizing the vircator efficiency. For the experiments reported on, the vircator was driven by a 500 kV/500 J compact Marx generator that can be operated repetitively at 10 Hz. For these experiments, the Marx generator was operating in single shot mode. During the experiments presented here, a couple of geometrical features were varied as well as the applied voltage. Their influence on the radiated field strength, dominating frequency and bandwidth are reported on and discussed.

Abstract [en]

One method to characterize the radiated microwave field from a high-power microwave (HPM) source is to measure the radiated high-level electromagnetic field in several locations at a high sampling rate registering the frequency time dependence, thus being able to determine the radiated pattern and mode. A complete free-field measurement system for measuring the magnetic field component in high-level electromagnetic fields has been developed at FOI. The system consists of a B-dot sensor and a balun, both designed and constructed at FOI. The B-dot sensor is designed as two cylindrical loop sensors with differential output. The balun is a microstrip design etched on a dual sided PTFE circuit board. Complete systems have been calibrated at SP Technical Research Institute of Sweden. A method to analyze the data from the free-field systems has been developed.